The invention relates to automatic transmissions for carrying out automatic shifting by switching power transmission lines, which may be in turn controlled by the operation of hydraulic clutches.
Automatic transmissions are so constituted as to obtain desired driving characteristics by automatically changing their speed ratios in accordance with the driving conditions. In order to attain this purpose, it is customary to provide a shift map composed of upshifting and downshifting lines for each speed range, the lines being established in relation to the vehicle speed and the engine power output, and to control the automatic transmission to shift the speed ranges according to the shifting map dependent on the changes of traveling states as indicated on the shift map. One example of such shifting control is disclosed in Japanese Laid-Open Patent Publication No. 61-189354, for example.
Hydraulic clutches used for shift control are controlled by supplying or discharging working fluid of a given pressure, and have been widely used for setting a speed in an automatic transmission, as seen in Japanese Patent Publication 52(1977)-21131.
In changing the speed in an automatic transmission, the hydraulic pressure in an hydraulic chamber of an engaging hydraulic clutch (hereinafter referred to as "pre-shift clutch") is discharged into a drain and at the same time fluid of the controlling pressure is fed into the hydraulic chamber of the hydraulic clutch for a gear train to be set up (hereinafter referred to as "post-shift clutch"). The changes in hydraulic pressure in the pre-shift and the post-shift clutches may be illustrated as in FIG. 8. In this figure a shift is started at time t.sub.1. The hydraulic pressure in the hydraulic chamber of the pre-shift clutch rapidly drops from time t.sub.1, although it exhibits a plateau pressure for some time owing to the functions of the accumulator, orifice control valve, and the like. On the other hand, the pressure in the hydraulic chamber of the post-shift clutch is rapidly stepped up to P.sub.2 at time t.sub.2 after it is maintained at P.sub.1 from time t.sub.1 to t.sub.2 ; between time t.sub.2 and t.sub.3 retained on a plateau gently going up from P.sub.2 to P.sub.3 ; and increased up to the predetermined clutch pressure P.sub.L at the end of the time interval between t.sub.3 and t.sub.4. Such hydraulic pressure control is exercised to ensure smooth engagement of the post-shift clutch so that the shift shock is reduced, for which the change in hydraulic pressure has been conventionally obtained by the accumulator disposed in the hydraulic line supplying the fluid to the post-clutch.
In FIG. 8, the hydraulic pressure change in the post-shift clutch (line A ) from time t.sub.1 to t.sub.2 indicates the pressure change observed over the period from the time that the piston of the post-shift hydraulic clutch begins to move to the time that the piston actually begins to push the clutch plate, i.e. the period required for the piston to move the clutch clearance. Since the motion of the piston during this period is dead stroke and the piston moves without doing any work, the hydraulic pressure in the hydraulic chamber supplies a low pressure needed for the motion, as exhibited in the time interval between t.sub.1 to t.sub.2 in the Figure.
Because of this, although the period actually required for the hydraulic clutch to carry out the shifting is from t.sub.2 to t.sub.4 shift time T.sub.2 is prolonged by the dead stroke period (t.sub.1 to t.sub.2), causing a disadvantage that the driving feeling is degraded.
This problem may seem to be solved by maximizing the flow rate of the control fluid supplied to the post-clutch at almost the same time as the initiation of the shifting to minimize the dead stroke moving period of the piston. In this case, however, if the control fluid flow rate is maintained maximum, the rise in the hydraulic pressure over the period t.sub.1 to t.sub.4 of FIG. 8 is so great that it will cause a shift shock. Thus, it is necessary to estimate the period of the dead stroke of the piston by the use of a timer, and reduce the amount of the control fluid to be supplied at the end of the period, so that the pressure change over the period t.sub.2 to t.sub.4 as shown in FIG. 8 may be obtained.
However, the dead stroke moving time (t.sub.1 to t.sub.2) is easily affected by the variation of the individual clutch clearances, so that the above estimated time for the dead stroke movement may well vary from one transmission to another. Therefore, with the timer for maximizing the amount of the supplying control fluid over the estimated period, the piston may move more than the dead stroke and press the post-shift clutch, which will result in a sudden engagement of the post-shift clutch and a disadvantageous shift shock.
In conventional transmission control for power on shift-down (i.e. shift down operation while depressing the accelerator pedal, e.g. kick-down) or for power-off shift-up (i.e. shifting up operation by releasing the accelerator pedal during driving), the pre-shift clutch (which has been so far engaged) is disengaged and thereafter the post-shift clutch (which is to be engaged in the shifting) is engaged when the input rotation becomes in synchronism with the output rotation. Such control has an advantage that the engagement is smooth since no inertial energy is then exchanged between the input and output shafts.
In the conventional control, however, the accuracy of the synchronization timing of the input and output rotations in the post-shift clutch is low, since the detection (determination) of the synchronization resorts to a timer or the relationship between the vehicle speed and the engine speed, which are affected with the fluid temperature individual variations, and slips in the torque converter or the fluid couplings. Inaccurate determination of the synchronization timing will lead to, for example, an earlier engagement of the post-shift clutch which causes a shift shock, or a delayed engagement of the post-shift clutch which cases an excessively rapid increase in engine speed or unplesant feelings.
Another problem is also involved in the conventional transmission control in which the synchronization timing of the input and output rotations is detected for the post-shift clutch and the working fluid is supplied to the post-shift clutch, in that the time of engagement of the clutch delays due to the fact that the clutch begins to engage only after the piston has moved the dead stroke distance. This drawback may be improved by giving a shift signal at a time earlier than the true synchronization time. However, a difficulty still remains that the dead stroke greatly differs for individual transmissions and accurate detection of the synchronization time in prior to the synchronization is not easy.